地球最老礦物源自何方？

原文網址：www.sciencedaily.com/releases/2016/04/160428173233.htm

Origin of Earth's oldest crystals

地球最老礦物源自何方？

New research suggests that the very oldest pieces of rock on Earth -- zircon crystals -- are likely to have formed in the craters left by violent asteroid impacts that peppered our nascent planet, rather than via plate tectonics as was previously believed. Rocks that formed over the course of Earth's history allow geologists to infer things such as when water first appeared on the planet, how our climate has varied, and even where life came from. However, we can only go back in time so far, as the only material we have from the very early Earth comes in the form of tiny, naturally zircon crystals.

最新的研究提出地球上最老的岩石碎片—即鋯石的晶體，可能是小行星群猛烈轟炸初生地球後，在留下來的隕石坑內部形成，而非先前認為的是由板塊運動產生。在地球歷史漫漫軌跡中形成的諸多岩石可以讓地質學家從中推論出種種事物，像是地球上何時出現第一滴水，我們的氣候如何變化，甚至是生命的源頭。然而，至今我們僅能追溯自唯一從地球相當早期遺留下來的物質，也就是這些自然產生的微小鋯石結晶形成之時。

Naturally then, the origin of these crystals, which are approximately the width of a human hair and more than four billion years old (the Earth being just over four and a half billion years old), has become a matter of major debate. Fifteen years ago these crystals first made headlines when they revealed the presence of water on the surface of the Earth (thought to be a key ingredient for the origin of life) when they were forming.

自然而然地，這些僅約人髮寬，年代超過40億年的礦物起源為何，之後便成為一項眾說紛紜的議題。15年前，這些礦物因為顯示出在它們形成的時候地球表面已經有水(被認為是生物起源所需的關鍵要素之一)而首度聲名大噪。

Ten years ago, a team of researchers in the US1 argued that the ancient zircon crystals probably formed when tectonic plates moving around on the Earth's surface collided with each other in a similar fashion to the disruption taking place in the Andes Mountains today, where the ocean floor under the Pacific Ocean is plunging under South America.

10年前，一群來自US1的研究人員聲稱這些古老的鋯石結晶可能形成於板塊在地球表面移動而彼此碰撞時，就像今日太平洋底部的海床俯衝至南美洲之下，而在安地斯山脈內部發生的劇烈變動一般。

However, current evidence suggests that plate tectonics -- as we know it today -- was not occurring on the early Earth. So, the question remained: Where did the crystals come from?

然而，現有的證據顯示出我們今日熟悉的板塊構造運動在地球早期還尚未發生。因此問題仍然懸而未解：這些結晶究竟來自何方？

Recently, geologists suggested these grains may have formed in huge impact craters produced as chunks of rock from space, up to several kilometres in diameter, slammed into a young Earth. To test this idea, researchers from Trinity College Dublin decided to study a much younger impact crater to see if zircon crystals similar to the very old ones could possibly have formed in these violent settings.

最近，地質學家提出的一則新說法認為由半徑最高可達7公里，從太空來的岩石群重擊年幼地球時，所產生的巨大撞擊坑中形成了這些礦物顆粒。為了證實他們的想法，都柏林聖三一大學的研究人員決定研究年輕許多的撞擊坑，以觀察在這種嚴酷的環境當中，是否會產生跟那些相當古老的鋯石十分相似的鋯石結晶。

In the summer of 2014, with the support of the Irish Reseach Council (IRC) and Science Foundation Ireland (SFI), the team collected thousands of zircons from the Sudbury impact crater, Ontario, Canada -- the best preserved large impact crater on Earth and the planet's second oldest confirmed crater at almost two billion years old.

2014年夏季，在愛爾蘭研究學會和愛爾蘭科學基金會的協助之下，研究團隊從加拿大安大略省的索德柏立(Sudbury)撞擊坑中採集了數以千計的鋯石。這個撞擊坑是地球上被研究得最為透徹的大型撞擊坑，同時也是已確認年代的撞擊坑中第二古老者，年代將近20億年。

After analysing these crystals at the Swedish Museum of Natural History in Stockholm, they discovered that the crystal compositions were indistinguishable from the ancient set.

在斯德哥爾摩的瑞典自然史博物館中詳盡分析這些晶體後，他們發現這些晶體的成分幾乎無法跟那些古老的鋯石區分開來。

PhD Researcher in Trinity's School of Natural Sciences, Gavin Kenny, is first author of the article which explains these findings, and which has just been published in leading international journal, Geology.

此篇論文的第一作者，聖三一大學自然科學院的博士後研究員 Gavin Kenny解釋了他們剛刊登於國際頂尖期刊《地質》(Geology)上的新發現。

He said: "What we found was quite surprising. Many people thought the very ancient zircon crystals couldn't have formed in impact craters, but we now know they could have. There's a lot we still don't fully understand about these little guys but it looks like we may now be able to form a more coherent story of Earth's early years -- one which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times."

他說：「我們發現的事物相當令人驚訝。許多人認為年代十分久遠的鋯石不可能在撞擊坑中形成，但現在我們知道其實它們可以。對於這些小傢伙我們還有許多尚未完全了解的地方，但現在看起來我們可以將地球早年的故事拼湊得更加完整。我們的發現跟現行的想法相當符合，認為早期地球飽受小行星的頻繁轟炸，與較為近期相比頻率高出許多。」

Gavin Kenny recently travelled to the annual Lunar and Planetary Science Conference (LPSC) in Houston, Texas, to present these findings to the space science community.

Gavin Kenny近日前往於休士頓舉行的年度月球和行星科學會議，向天文學社群發表他們的發現。

He added: "There was a lot of enthusiasm for our findings. Just two years ago a group2 had studied the likely timing of impacts on the early Earth and they suggested that these impacts might explain the ages of the ancient zircons. They were understandably very happy to see that the chemistry of the zircons from the Canadian impact crater matched the oldest crystals known to man."

他補充說：「有很多人對我們的新發現抱持著熱切關注。就在兩年前有另外一個團隊研究了同樣形成於早期地球，年代相近的撞擊坑，並提出這些撞擊事件或許可以用來解釋遠古鋯石的年代。可以想見他們會很樂意看到從加拿大的撞擊坑中取得的鋯石，其化學成分跟這些人類所知最古老的晶體竟然如此吻合。」

引用自：Trinity College Dublin. "Origin of Earth's oldest crystals." ScienceDaily. ScienceDaily, 28 April 2016.